This invention relates generally to hydraulic pumps and axle driving apparatus and, more particularly, to a system for electric actuation of such devices, including electronic circuitry to automatically drive the axle driving apparatus to a neutral position.
Hydraulic pumps, transaxles, hydrostatic transmission assemblies (“HSTs”) and integrated hydrostatic transaxles (“IHTs”) are known in the art. Generally, these devices include an end cap or a center section on which is mounted a rotating hydraulic pump and, in some applications, a rotating hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons, which are in fluid communication through hydraulic porting formed in the center section or through hoses to a separate hydraulic motor. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces an operating oil through the hydraulic porting or hoses to the hydraulic motor to move the motor pistons. The axial motion of the motor pistons causes the hydraulic motor to rotate as the motor pistons bear against a thrust bearing. In this manner, the rotation of the hydraulic motor may be used to drive the vehicle axles of a riding lawn mower, small tractor and the like. Separate hydraulic motors such as geroller, radial piston, and gerotor are also known and similarly function to drive a motor output shaft or one or more axles.
To adjust the speed and direction of rotation of the hydraulic motor and, accordingly, the speed and direction of rotation of the vehicle axles, the position of the swash plate with respect to the hydraulic pump pistons may be changed. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction. Additionally, the angle at which the swash plate addresses the hydraulic pump pistons can be changed to increase or decrease the amount of operating oil that is forced from the hydraulic pump to the hydraulic motor to change the speed at which the hydraulic motor rotates.
For use in changing the position of the moveable swash plate, it is known to include a trunnion arm that is coupled to the swash plate. A speed change lever or a speed change pedal is, in turn, coupled to the trunnion arm through a series of rods and levers or other driving link. In this manner, movement of the speed change lever/pedal results in movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle. Examples of such mechanisms for adjusting the speed of a vehicle may be seen in U.S. Pat. Nos. 6,122,996 and 5,819,537, which are incorporated herein by reference in their entirety. While these mechanisms for adjusting the speed of a vehicle have worked for their intended purpose, they require additional linkage, which limits the flexibility and ease of transaxle installation, and are more difficult for operators to control because of the control moments associated with the additional linkage.
For placing the swash plate in a position that neither affects the speed nor the direction of rotation of the hydraulic motor, i.e., the neutral position, some hydraulic pumps or hydraulic transaxles provide a return to neutral mechanism that is normally implemented as an integral part of the vehicle linkage. While these return to neutral mechanisms work for their intended purpose, they do suffer disadvantages. For example, these known return to neutral mechanisms require complex, costly linkages that require substantial assembly time. These known mechanisms also fail to allow for flexibility with respect to the type and orientation of driving linkages that may be used in connection with the hydraulic pump.